Wireless communication systems, i.e., systems that provide communication services to wireless communication devices such as mobile phones, smartphones etc. (often denoted by UE that is short for user equipment), have evolved during the last decade into systems that must utilize the radio spectrum in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train.
To meet this demand, within the third generation partnership project, 3GPP, work is being done regarding possible enhancements to radio resource management, RRM, performance in high speed train environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) which vehicles travel at greater than 300 km/h and where there is demand for using mobile services. In a motivation contribution to 3GPP RAN #66, RP-141849, four scenarios of interest to wireless communication network operators are disclosed. In a number of these scenarios, there is a dedicated network to provide railway coverage of the cellular system; either as a standalone network, or used in conjunction with a public network which is not specifically designed to provide high speed train coverage.
For the development of the fifth generation of mobile telecommunication technology (5G), the International Telecommunication Union (ITU) has defined a set of requirements, International Mobile Telecommunications (IMT)-2020, which includes the support of UE speeds of above 500 km/h with respect to mobility and data communication.
In the standardization of the 3GPP Release 13 study item on improved performance for UE on-board high-speed trains contributions have been made relating to a new network deployment scenario—Unidirectional remote radio head (RRH) arrangement—that will allow long term evolution (LTE) UEs to meet the high speed requirements in IMT-2020 (above 500 km/h). Particularly speeds up to 750 km/h has been investigated—see for instance 3GPP meeting documents R4-155743 and R4-155752.
Apart from the relatively shortened time for detecting suitable neighbour cells for handover or cell reselection, high speed movement of the UE may also lead to significant Doppler shifts of the received radio signals. Such a Doppler shift forces the UE to increase its demodulation frequency when moving towards a cell (i.e. moving towards an antenna that defines a radio lobe of the cell), and decrease demodulation frequency when moving away from a cell, in order to maintain an acceptable receiver performance.
The Doppler shift can be expressed as:
      Δ    ⁢                  ⁢    f    =      f    (                                        1            -                          v              c                                            1            +                          v              c                                          -      1        )  
where c is the speed of light and v is the relative velocity of the UE towards the transmitting antenna. Referring to FIG. 1, an UE 101 is on a high speed train 103 on a railway track 104, connected to and moving away from cell A2 105 and quickly needs to detect cell B1 107 towards which the UE 101 is moving with a velocity νUE 109 of the train. According to current standard an antenna 111, 113 of a cell site can be as close as 2 m from the railway track 104, mainly motivated by that the wireless communication network would be integrated with the high-speed railway infrastructure. With an angle α between railway track 104 and a direction 106 to a cell antenna 113 and a UE velocity νUE, the relative velocity ν towards the transmitting antenna giving rise to Doppler shift is ν=νUE cos α.
The magnitude of the Doppler shift depends on the relative velocity of the UE 101 towards the transmitting antenna in a cell. Consequently, with transceivers located close to such a constrained path along which an UE is moving along a railway track, i.e., a small angle between the trajectory of the UE and the line between the UE and the transmitting antenna, a substantial part of the UE velocity will transfer into a Doppler shift. Moreover there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna and the smaller the angle, the more abrupt is the change from positive to negative Doppler shift.
Each radio propagation path may have its own Doppler shift, depending on how the radio waves travel between the transmitting antenna and the UE. In case of line-of-sight there is one dominant path, whereas in e.g. urban areas there is generally scatter (reflections) due to buildings to which the UE has a relative velocity, giving rise to multiple paths for the signal to propagate to the UE, each with a different Doppler shift. Since the received signal (in general) is the superposition of those paths, it gives rise to Doppler spread which degrades radio receiver performance by smearing out the signal in the frequency domain hence causing inter-carrier interference.
High-speed railway track sections are generally using dedicated platforms often elevated above the landscape or city beneath. Hence, there are few objects that can cause a significant Doppler spread; with cell sites located along the track line-of-sight will be dominating at least between the cell site and the train. Moreover, in built-up areas as well as when a train is approaching or passing stations the speed is generally restricted of concern for public safety and disturbing noise, and as a consequence the Doppler shift becomes small.
However, there remain a number of challenges in relation to high-speed train scenarios in prior art. For example: in case UEs with different frequency offset characteristics have their PUCCH scheduled in the same resource block pair the orthogonality of the cover codes used for multiplexing of several sets of UEs will be broken and PUCCH decoding performance will decrease due to interference.
The Unidirectional RRH arrangement allows UEs that are traveling at high speed to maintain a downlink modulation frequency with stable frequency offset caused by the Doppler. As a result a UE in such arrangement achieves a good downlink performance. However, the system performance depends not only on the downlink but also on the uplink performance. For securing an overall good performance on system level it is important that limiting factors on the uplink are addressed and solved. One such factor is the PUCCH decoding performance.
In case it is not clear from the context in which they appear, below follows a summary of abbreviations of some of the technical terms used in the description above.
Abbreviation Explanation
ACK Acknowledged
BPSK Binary phase-shift keying
CCE Control channel element
CSI Channel state information
CQI Channel quality indication
FFT Fast Fourier Transform
FDD Frequency division duplex
HARQ Hybrid automatic repeat requestHST High speed train
LTE Long term evolution
MIMO Multiple input multiple output
MME Mobility management entity
MU-MIMO Multi-user MIMO
NACK Not acknowledged
PDCCH Physical downlink control channel
PRACH Physical random access channel
PUCCH Physical uplink control channel
PUSCH Physical uplink shared channel
QPSK Quadrature phase-shift keying
RRC Radio resource control
RRH Remote radio head
RRU Remote radio unit
SFN Single frequency network
SR Scheduling request
TDD Time division duplex
UE User equipment
UL Uplink